Porous, microspheroidal, nuclear fuels having internal porosity

ABSTRACT

A process is described for the preparation of microspheroidal particles comprising absorbing uranium cations from a uranyl aqueous solution onto small spherical ion exchange resin particles, drying the uranium-loaded resin, and then carbonizing the dried resin to form a porous carbon spheroid having closed porosity with an oxide or carbide or uranium uniformly dispersed within its volume. The same general technique can be used to form microspheres of the oxides or carbides of other actinide metals.

ite Sites Patet Googin et a1.

POROUS, MICROSPHERGIDAL, NUCLEAR FUELS HAVING INTERNAL POROSHTY inventors: John M. Googin; Charles R.

Sehmitt, both of Oak Ridge, Tenn.

Assignee: The United States oi America as represented by the United States Energy Research and Development Administration, Washington, DC.

Filed: (let. 16, 1972 Appl. No: 298,134

Related U.S. Application Data Division of Ser. No. 48,579, May 25, abandoned.

US. Cl. 252/3013 R; 176/67; 176/91 SP; 264/05; 264/29 Int. Cl G2lc 3/02 Field of Search 252/301.l R; 176/67-69, 91 SP; 264/05, 29

References Cited ILJNITED STATES PATENTS 7/1963 Martin 252/301.1 R

[ June 3, 1975 3,403,003 9/1968 Hamling 423/251 3,433,749 4/1969 Lonadier et a1. 423/251 3,649,452 3/1972 Chin et al. 176/68 3,673,101 6/1972 McKenney 6M1... 252/3011 R 3,764,550 10/1973 Block et a1. 252/3011 R Primary Examiner-Benjamin R. Padgett Assistanz ExaminerR. E. Schafer Attorney, Agent, or Firm-John A. Horan; David S. Zachry; Irving Barrack [5 7 ABSTRACT 13 Claims, N0 Drawings 1 POROUS, MICROSPHEROIDAL, NUCLEAR FUELS HAVING INTERNAL POROSITY BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

This is a division of application Ser. No. 48,579, filed May 25. 1970, now abandoned.

The present invention relates to a novel nuclear fuel composition characterized as microspheroidal particles consisting of a porous carbon matrix and an oxide or carbide of uranium uniformly dispersed within the volume of said matrix.

In the ideal sense, a model fabrication process for a nuclear fuel is one which is characterized by simplicity, economy of operation, and by its amenability to reproduce a nuclear fuel which meets product requirements for use in a power-producing reactor. In addition, it is most advantageous if the fuel is so constructed that it can readily be reprocessed for recovery of fissile and fertile values after its discharge from a going reactor.

It is accordingly the principal object of this invention to provide a nuclear fuel and a process for its fabrication which meets these aforementioned criteria.

SUMMARY OF THE INVENTION In its product aspect, this invention is concerned with a microspheroidal particle having a particle size in the range 5 to 2,000 microns. It consists essentially ofa porous carbon or graphite matrix containing a homogeneously dispersed phase consisting of an oxide, carbide, nitride, or silicide of a fissionable or fertile metal or alloy thereof. A unique feature of the porous matrix is that its porosity is mainly of the closed-pore type, of the order of 30 volume per cent, as measured metallographically. A particular advantage of the microspheroidal particles of this invention is the fact that a considerable percentage of the fissionable or fertile values are found within the closed pores of the matrix which serve as a convenient trap for gaseous fission products generated during the course of the fission process. In instances where higher degrees of fission product retention are required, the spheroidal particle can be coated with pyrolytically deposited carbon or carbide layers to develop essentially complete gas impermeability. When a fuel loading of the microspheroidal particles of this invention has reached its useful life, it is readily removable from the reactor and can be processed by well known chemical reprocessing techniques to recover fissionable and fertile values as well as useful fission products. Such reprocessing in one suggested scheme would merely involve oxidation of the carbon or graphite matrix in air or oxygen at an elevated temperature sufficient to gasify the carbon, leaving a residue readily dissolvable in aqueous acidic media to produce a feed solution from which the dissolved fissionable and fertile values can be readily separated by well known solvent extraction techniques.

In its process aspect, the present invention is concerned with a process for the preparation of microspheroidal particles of the character described which comprises immersing or contacting spheroidal organic ion exchange resin particles or sorbing selected metal cations on an organic ion exchange resin to a desired loading, drying the loaded resin particles, and then carbonizing the dried resin. Among the ion exhange resins useful in making the porous microspheroidal particles of this invention are cation exchange resins containing sulfonic, phosphonic, or carboxylic acid as the functional group at sites along a polymeric backbone chain and anion exchange resins containing quaternary ammonium, primary, secondary, and tertiary amine groups along a backbone polymer chain. Sorption on anion exchange resins occurs from acidic solutions such as aqueous solutions of nitric, sulfuric, or carbonic acid.

Among the particular commercial ion exchange resins useful in the process of this invention are Dowex 1, Dowex 21K, Dowex 0X12, and Dowex 50W, although ion exchange resins from other commercial vendors would be equally satisfactory. Dowex I and Dowex 21K are anion exchange resins incorporating a quaternary ammonium type of structure in which the four substituents on the nitrogen atom are a polymeric benzyl and three methyl groups.

Dowex 50W is a strongly acidic cation resin made by the nuclear sulfonation of styrene-divinylbenzene beads. It may be designated chemically as R.SO I-I, where R represents the polystyrene resin.

The degree of crosslinkage in an ion exchange resin bead refers to the fraction of divinylbenzene it contains. For the Dow Chemical Company resins, the per cent crosslinkage is indicated by a number following an X" after the name of a particular resin. Thus, Dowex 1X1 resin is made from a copolymer containing 1 per cent divinylbenzene and Dowex 1X10 resin would contain 10 per cent divinylbenzene. The divinylbenzene content contributes the third dimension to the polymer network and makes it insoluble. The very low crosslinked resins are highly swollen, allow faster diffusion of ions within the resins, and are soft and easily deformed. The higher crosslinked (10 per cent) resins are harder and do not swell as much when exposed to solutions.

The conditions under which a desired degree of metal cation loading can be obtained in a reproducible manner are well known in the available literature. See, for example, Anion Exchange of Uranium in Nitrate Solutions, Journal of Chemical and Engineering Data, Vol. VI, No. 2, April 1961, p. 217, etc.

After loading has been accomplished to the extent desired, the loaded resin particles are then separated from the solution either by decantation or filtration. Excess liquid is removed by washing and the separated resin particles are then dried to remove excess moisture. The loaded resin particles should be thoroughly dried at noncarbonizing temperatures. If they are not, there is a risk that subsequent carbonization will cause cracking or decrepitation of the particles as the resin is converted to carbon or graphite. Drying to remove moisture can be effected at a temperature of about 110C.

The dried, metal-loaded ion exchange resin spheres are then subjected to a carbonization reaction by heating slowly in an inert atmosphere to a temperature adequate to convert the resin spheres to carbon using a controlled heating cycle. Normally, a temperature of 900l C. is adequate to accomplish carbonization. A satisfactory heating rate is one which allows gases from the internal volume of the metal-loaded resins to be dispelled without excessive internal particle deformation, decrepitation, or cracking of the particles. In order to maintain the sphericity of the particles, it is preferred that the particles be carbonized in a fluidized bed using an inert gas as the fluidizing medium.

In describing this process for making microspheroidal particles it should be made clear that the sphericity beads was then heated in flowing helium for 3 hours at at least 1,700C. and not more than 2,200C. in a carbon induction furnace. No significant shrinkage was noted as comparable to that which took place during desired in the final product is a function of the degree 5 carbonization. of sphericity of the unloaded resin beads, and the sub- Photomicrographs of cross sections of the carbonized sequent processing involved in converting the metaluranium-loaded beads after 950 and 2,200C. heat loaded beads to carbon or graphite. These and other treatments showed the carbonized beads to contain inspecific aspects of the invention discussed heretofore in ternal porosity or fissures. Cross sections of Dowex general terms will be made clearer in the specific em- 21K resin beads before and after uranyl nitrate adsorpbodiments to be described in the following examples. tion have been examined to show that the internal voids EXAMPLE] and fissures are not inherent in the anion exchange resin structure before carbonization.

A batch of Dowex 1X1 (50-100 mesh) anion ex- X-ray diffraction studies of the uranium-loaded resin change resin was converted to the nitrate form by treatafter carbonizing at 950C. and high firing at 1,400", ment with 0.9 M NH NO 0.1 M HNO solution in 1,700", and 2,200C. under flowing helium were made an ion exchange Column until th 0011mm fflu n was in order to identify the chemical form of the uranium free Of chloride ion. The resin was then rinsed with diS- associated with the carbon matrix at these various firtilled water. Three 100-gram portions of the resin were ing temperatures. A comparison of the X-ray diffracloaded with uranyl nitrate by equilibrating with 800 ml tion patterns obtained with U0 UC, graphite, and of 6.0 M NH NO solution containing sufficient dis U 0 standards showed that at l,400C. the uranium is solved uranyl nitrate to provide various uranium loadpresent as almost all U0 At l,700C., the diffraction ings of 50 to 150 g U/Kg dry resin according to predepattern of the coked resin shows the presence of U0 termined equilibrium relationships. The resins were and UC, with some carbon. A comparison of the X-ray then filtered, air dried, dried in an oven at 100C, and diffraction patterns of the resin high fired at 2,200C. carbonized under flowing helium. One l00-gram porwith standard patterns of uranium monocarbide and tion was carbonized at 500C, another at 700C, and uranium dicarbide showed the presence of both UC a third at 850C. Each portion was carbonized for 1 and UC hour. The analytical results of these three portions are A summary of the uranium content, carbon content, shown in Table l. and mercury displacement density for uranium-loaded TABLE I carbonization of Uranium-Loaded Ion Exchange Resin Initial carbonization Sample Uranium Temperature Coke Analysis Nitrogen No. Loading" (C.)" Color gU/g 71 Carbon ppm lEC-l 56.5 500 Brown 0.224 55.16 360 [EC-2 92.1 700 Black 0.3582 47.48 243 [EC-3 135.6 850 Black 0.4486 38.79 I89 "Gram uranium pcr Kg dry resin.

I.() hour soak time in flowing helium.

Spectrographic analyses of pulverized coke product Dowex 21K resin beads after heating under flowing hemesh) indicate that a high degree of uranium and lium at 950C., 1,700C., and 2,200C. is given in Table carbon purity is attained by this method. 11.

The foregoing example is included to show that vari- TABLE H able loadings of uranium can be attained, that some decomposition of the resin occurs at temperatures below 950C., but that complete COl'lVCI'SlOIl Of the resin to Properties of Coked, Uranium-Loaded lon Exchange Resin carbon requires higher carbonization temperatures. Hem Treatment Density EXAMPLE II g U/g Coke Carbon g/l A batch of Dowex 21 K 16-20 mesh) anion resin was 38% 8:338 loaded with uranyl nitrate by equilibrating with 6.0 M 2200 0.04318 95.14 0.980 ammonium nitrate solution containing sufficient uranyl nitrate to provide an initial uranium loading of 29 g /K d resin Th resin was fil d i d i d d These data indlcate that crystallization is rather 1ncarbonized under flowing helium for 1 hour at 950C. completfi at 950C as shown by the large increase in using a vertical quartz furnace. While a heating cycle X-ray line intensities upon high firing at 2,200C. of approximately 6 hours can be used to attain th tar- Cross-sectional examination at 1,000 magnification get carbonization temperature of 950C., a much of elongated and spherical beads after high firing at slower heating cycle is desirable to avoid large internal fissures that may result from dispellation of volatiles at too rapid a rate. The batch of 950C.-carbonized resin 2,200C. showed the uranium carbide homogeneously dispersed in the solid portion of the carbonized bead as pepper-like grains.

EXAMPLE 111 One pound of Dowex 50W cation exchange resin microspheres, having 50-100 mesh size, was soaked for 16 hours in a uranyl nitrate solution. This solution was prepared by dissolving U 0 in 8 N nitric acid. Subsequent dilution with distilled water produced an approximate 0.5 N U0 (N0 solution.

The loaded resin microspheres were dried at 1 C. for 16 hours after which they were carbonized in a fluidized bed using helium as the fluidizing medium. Carbonization was accomplished at 1,000C. using a maximum temperature rise of 200C/hr.

The resultant uranium-loaded microspheres contained about 22 per cent uranium by weight in the form of USO, as indicated by X-ray diffraction analysis. (The sulfur was present because sulfonated cation exchange resins were used. At 1,600C., uranyl nitrate on sulfonated cation resins converts to UO UC as indicated by X-ray diffraction analysis. Compounds of uranium and oxygen would result if the carboxylic acid form of cation exchange resin were used.) They had a density of about 1.9 g/cc and an open porosity of about 40 per cent. The microsphere products were further characterized by metallography and microradiography. These techniques showed that a uniform distribution of uranium in the microsphere was achieved.

Uranium-containing microspheres prepared as described in Example 111 were coated with pyrolytic carbon to give either an interrupted duplex coating or a monolayer coating. For the interrupted duplex coating, a -micron-thick, low-density coating (about 1.6 g/cc) was applied using propylene in a fluidized bed at 1,250C. This was followed by the application of a 50- micron-thick, high-density layer (about 1.8 g/cc) also using propylene but increasing the temperature to 1,900C.

Monolayers of both 60-micron and 80-micron thickness were applied to some of the microspheres in a fluidized bed using propylene at 1,400C. These coatings had a density of about 1.75 g/cc.

EXAMPLE IV A batch of Dowex 50Xl2 cation exchange resin (100-200 mesh spheres) in the hydrogen form was contacted with uranyl sulfate solution (approximately 200 gU/liter) for 48 hours using magnetic stirring. The resin was filtered, rinsed rapidly with distilled water to remove any residual mother liquor, and dried at 1 10C. This treatment provided a resin uranium loading of 7.32 grams uranium per kilogram of dry resin. After carbonizing with helium at 900C, the resin spheres analyzed 1.85 per cent uranium, 79.18 per cent carbon, and 6.08 per cent sulfur. By subsequent high firing to temperatures of l,800C. or higher in an inert atmosphere, the sulfur can be essentially completely removed.

In the examples, the process of forming uraniumcontaining microspheroidal particles is provided as a specific embodiment. It should be understood, however, that the general technique described in forming uranium-containing microspheres can be adapted for making actinide oxideor carbide-containing microspheres of plutonium and thorium or mixtures of uranium, thorium, and plutonium. In addition, it should be noted that alloys of the actinides can also be formed as inclusions within the internal volume of the carbonized or graphitized matrix by appropriate loading of the resin. For example, where it is desired to form a microsphere containing a uranium-zirconium alloy, the resin is loaded with uranium and zirconium salts in the proportions corresponding to the desired alloy composition and then the process of microsphere formation proceeds by the appropriate carbonization or graphitization reactions.

What is claimed is:

1. A microspheroidal particle consisting essentially of a porous matrix of carbon or graphite, a substantial portion of the porosity of said matrix being of the closed-pore type, and an oxide or carbide of a metal selected from the group uranium, plutonium, and thorium within the internal volume and closed pores of said matrix.

2. The particle of claim 1 and at least one pyrolytic carbon coating deposited on the surface of said particle.

3. The particle of claim 1 wherein the matrix is graphite and the selected metal is uranium distributed within said matrix as a uranium carbide.

4. The particle of claim 1 wherein the matrix is carbon and the selected metal is a mixture of uranium and thorium distributed within said matrix as their respective carbides.

5. The particle of claim 1 wherein the matrix is carbon and the selected metal is a mixture of uranium and plutonium distributed within said matrix as their respective carbides.

6. Microspheres of uranium carbide produced by loading an ion exchange resin containing ion exchange sites for sorbing uranyl ions, drying the loaded resin and then heating said dried resin in an inert atmosphere at a temperature in the range 1,400-2,200C. to convert the resin to a porous carbon matrix containing closed pores and a homogeneously dispersed uranium carbide phase. 1

7. The particle of claim 6 with at least one pyrolytic carbon coating deposited thereon.

8. The particle of claim 1 wherein the matrix comprises carbon and the metal carbide within said matrix is a uranium carbide.

9. The particle of claim 1 wherein the matrix is carbon and U0 within said matrix.

10. The particle of claim 1 wherein the matrix comprises carbon and a mixture of a uranium carbide and a uranium oxide within said matrix.

1 l. The particle of claim 1 wherein the matrix is carbon and a mixture of U0 and UC within said matrix.

12. A microspheroidal particle 5 to 2,000 microns in diameter consisting essentially of a porous carbon matrix and a compound selected from the group consisting of an oxide and a carbide of at least one metal selected from the group consisting of uranium, plutonium, and thorium distributed within said matrix.

13. The particle of claim 12 with at least one pyrolytic carbon coating deposited thereon. 

1. A MICROSPHEROIDAL PARTICLE CONSISTING ESSENTIALLY OF A POROUS MATRIX OF CARBON OR GRAPHITE, A SUBSTANTIAL PORTION OF THE POROSITY OF SAID MATRIX BEING OF THE CLOSED-PORE TYPE, AND AN OXIDE OR CARBIDE OF A METAL SELECTED FROM THE GROUP URANIUM, PLUTONIUM, AND THORIUM WITHIN THE INTERNAL VOLUME AND CLOSED PORES OF SAID MATRIX.
 1. A microspheroidal particle consisting essentially of a porous matrix of carbon or graphite, a substantial portion of the porosity of said matrix being of the closed-pore type, and an oxide or carbide of a metal selected from the group uranium, plutonium, and thorium within the internal volume and closed pores of said matrix.
 2. The particle of claim 1 and at least one pyrolytic carbon coating deposited on the surface of said particle.
 3. The particle of claim 1 wherein the matrix is graphite and the selected metal is uranium distributed within said matrix as a uranium carbide.
 4. The particle of claim 1 wherein the matrix is carbon and the selected metal is a mixture of uranium and thorium distributed within said matrix as their respective carbides.
 5. The particle of claim 1 wherein the matrix is carbon and the selected metal is a mixture of uranium and plutonium distributed within said matrix as their respective carbides.
 6. MICROSPHERES HOF URANIUM CARBIDE PRODUCED BY LOADING AN ION EXCHANGE RESIN CONTAINING ION EXCHANGE SITES FOR SORBING URANYLIONS, DRYING THE LOADED RESIN AND THEN HEATING SAID DRIED RESIN IN AN INSERT ATMPSPHERE AT A TEMPERATURE IN THE RANGE 1,400*-2,200*C. TO CONVERT THE RESIN TO A POROUS CARBON MATRIX CONTAINING CLOSED PORES AND A HOMOGENEOUSLY DISPERSED URANIUM CARBIDE PHASE.
 6. Microspheres of uranium carbide produced by loading an ion exchange resin containing ion exchange sites for sorbing uranyl ions, drying the loaded resin and then heating said dried resin in an inert atmosphere at a temperature in the range 1,400*-2, 200*C. to convert the resin to a porous carbon matrix containing closed pores and a homogeneously dispersed uranium carbide phase.
 7. The particle of claim 6 with at least one pyrolytic carbon coating deposited thereon.
 8. The particle of claim 1 wherein the matrix comprises carbon and tHe metal carbide within said matrix is a uranium carbide.
 9. The particle of claim 1 wherein the matrix is carbon and UO2 within said matrix.
 10. The particle of claim 1 wherein the matrix comprises carbon and a mixture of a uranium carbide and a uranium oxide within said matrix.
 11. The particle of claim 1 wherein the matrix is carbon and a mixture of UO2 and UC within said matrix.
 12. A microspheroidal particle 5 to 2,000 microns in diameter consisting essentially of a porous carbon matrix and a compound selected from the group consisting of an oxide and a carbide of at least one metal selected from the group consisting of uranium, plutonium, and thorium distributed within said matrix.
 12. A MICROSPHEROIDAL PARTICLE 5 TO 2,000 MICRONS IN DIAMETER CONSISTING ESSENTIALLY OF A POROUS CARBON MATRIX AND A COMPOUND SELECTED FROM THE GROUP CONSISTING OF AN OXIDE AND A CARBIDE OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF URANIUM,LPLUTONIUM, AND THORIUM DISTRIBUTED WITHIN SAID MATRIX. 